In Third Generation Partnership Project (3GPP) Release 7, flexible RLC PDU size for High Speed Downlink Packet Access (HSDPA) is introduced to solve an RLC window stalling problem at a high air rate. The flexible RLC PDU size is also used to reduce protocol overhead and padding.
In 3GPP Release 8, an improved Layer 2 (L2) is introduced for High Speed Uplink Packet Access (HSUPA). MAC-i/is entity is responsible to segment a large RLC PDU into a plurality of small MAC PDUs in case of transmission failure or lack of radio resource in uplink. Further, it is proposed to use MAC segmentation functionality of the improved L2 to improve uplink coverage. In particular, an RLC PDU (e.g. a voice packet) is segmented into a plurality of small MAC PDUs according to a supportable transport format over an air interface. Then in a receiving side these small MAC PDUs are reassembled to recover the RLC PDU.
For example, the MAC segmentation functionality can be used in the following cases:
a) Case 1
                When there is a coverage problem, a data unit is segmented to fit radio conditions.b) Case 2        For an initial RLC transmission, a radio channel is good and a big RLC PDU is used. But in an RLC retransmission, the radio channel becomes bad and hence MAC segmentation of the retransmission may be beneficial in order to fit radio conditions.        
Generally, a precondition to recover an RLC PDU is that all segments from this RLC PDU should be correctly received. Hence, Outer Loop Power Control (OLPC) per segment will result in a higher Block Error Probability (BLEP) of the recovered RLC PDU than a target Block Error Rate (BLER) of the RLC PDU. For instance, if a target BLER for a voice packet is p, a BLEP of a recovered voice packet turns to p multiplied by m when this voice packet is segmented into m segments (note that error correlation is not considered). Therefore, in order to keep the recovered voice packet meeting the target BLER p, the target BLER of each of m segments should be p divided by m.
Currently a so-called composite OLPC, which is disclosed by 3GPP Tdoc R1-091736 entitled “Link Analysis of 2 ms TTI UL VoIP coverage”, has been proposed to ensure that a BLER of a recovered data unit meets a desired target BLER. The composite OLPC makes adjustments to a Signal-to-Interference Ratio (SIR) target based on whether an original data unit can be recovered or not after all segments from the original data unit are received. After several Hybrid Automatic Repeat reQuest (HARQ) transmission failures, the SIR target can be increased to a proper level and hence the recovered data unit can meet the desired target BLER.
Although the composite OLPC can make the recovered data unit meet the desired target BLER, it has a number of limitations. Firstly, the composite OLPC relates to only an uplink data channel, which fails to take into account a downlink data channel. Secondly, for a UE encountering a coverage problem, the increase of the SIR target is not favorable because the overhead of control channels is increased and the available power for data is reduced. Thirdly, since the SIR target is increased to the proper level only after several HARQ transmission failures, there is an extra packet loss/delay during the increase of the SIR target. Fourthly, if the data unit is not transmitted in segments any more, the SIR target has to be slowly decreased to a proper level, which means a radio resource waste.